Magnetic field generating assembly

ABSTRACT

A magnetic field generating assembly having at least two magnetic fields in the same sense. The generators are arranged such that, in use in a working region external to the generators, the resultant magnetic field exhibits a gradient extending in a first direction and at least second order variations in the fields in directions transverse to the first direction within the working region are substantially balanced such that an NMR experiment can be performed on a sample within the working region in accordance with the related method. The magnetic electrical field generators are substantially coaxial electrical coils, wherein the axis extends in the first direction.

BACKGROUND OF THE INVENTION

The invention relates to a magnetic field generating assembly and toapparatus for performing a nuclear magnetic resonance (NMR) experimentusing such an assembly.

In the field of nuclear magnetic resonance, it is necessary to generatewithin a working volume a substantially homogenous magnetic field toenable the NMR characteristics of molecules within the working volume tobe examined. Conventionally, this has been achieved by making use of acoil assembly at the centre of which an approximately spherical regionhaving a homogeneous magnetic field is generated. Such an assembly isdescribed in U.S. Pat. No. 4,656,423. This has proved satisfactory inthe case of NMR imaging of the human body and also NMR spectroscopy.However, there is a need to be able to perform NMR experiments onrelatively large bodies, for example polymer composites such as aircraftparts and the like and the cost of manufacturing a magnet of aconventional form into which the body can be placed is so large as to beprohibitive.

Samoilenko et al JETP Lett. Vol. 47, No 7, 10 Apr. 1988, describe howlarge field gradients within conventional high resolution NMR magnetscan provide sufficient field gradient to facilitate a selected region ofthe order of 0.1 mm with the application of an rf pulse only. However,again the working region is confined within the magnet.

Recently, attempts have been made to change the location of thehomogenous, working volume and in one such attempt described in U.S.Pat. No. 4,701,736 a magnet is described in which the sphericalhomogeneous region is projected outside the volume of the magnet. Anumber of nested, counter running coils are used. This suffers from thedisadvantages firstly that more power (in a resistive system) or moresuperconducting material (in a superconducting system) is required toproduce a given field strength in the working volume; and secondly theworking volume is spherical which is not particularly suitable forcertain applications.

A second assembly for generating a substantially homogeneous regionprojected from the assembly is described in EP-A-0186998 which comprisesa number of coil sets with their axes substantially parallel and spacedapart and this requires an undesirable amount of superconductor (in thecase of superconducting magnets) or power (in the case of resistivemagnets) to achieve a given field strength.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a magnetic fieldgenerating assembly comprises at least two magnetic field generators forgenerating magnetic fields having substantially coaxial portionsextending in the same sense, the generators being arranged such that inuse in a working region external to the generators the resultantmagnetic field exhibits a gradient extending in a first direction, andwherein at least the second order variations in the fields in directionstransverse to the first direction within the region are substantiallybalanced such that an NMR experiment can be performed on a sample withinthe working region.

In this assembly, it is possible to arrange for two or more magneticfield generators to generate substantially coaxial fields in the samesense and arrange for the second order variations in the field indirections transverse to the one direction to balance in the workingregion, thereby generating a relatively large working region spaced fromthe generators.

Although the magnetic field generators could comprise permanent magnets,preferably the generators comprise substantially coaxial, electricalcoils, most preferably the coils generating fields extending in thefirst direction. If superconducting coils are used, these could befabricated from the recently developed high temperature superconductingmaterials.

As will be explained below, it can be shown that the second (and fourth)radial derivatives of the magnetic fields exhibit changes in signdepending upon the radius of the coil. Consequently, by suitable choiceof coil radius it is possible to cancel at least the second (and higherradial derivatives) with a pair of coils.

In some examples, the centres of the coils will be offset axially.

Typically, the working region will be generally dish shaped although itcould also be substantially planar.

The dimensions of the working region and the degree of homogeneity indirections transverse to the first direction will depend upon the NMRexperiment being performed, the RF frequencies used, and thesignal-to-noise ratio desired. Typically, however, the homogeneitytransverse to the first direction will be better than 100 ppm.

In practice, the working region will have a significant dimension in thefirst direction so that effectively a set of homogeneous regions aregenerated along the first direction. This has the advantage that since agradient field is exhibited along the first direction, spatialinformation can be obtained from an object which intersects more thanone of the homogeneous regions.

As explained above, the assembly is particularly suitable for use inapparatus for performing an NMR experiment, the apparatus furthercomprising RF field generating means for generating an RF magnetic fieldin the working region; and detection means for detecting the resultantsignal echo. Typically, the apparatus will further comprise gradientfield generating means for imposing gradient fields within the workingregion to enable spatial information to be obtained.

The invention has many applications but is particularly suitable forobtaining NMR information from objects of relatively large size whichcannot be inserted into a conventional coil based system. For example,until now, no method has been suggested to allow the introduction ofpolymer composites on a commercial scale to be achieved. For example,aircraft frames and aerofoils are made of such polymer composites andthere is a need to provide as much information as possible both aboutmanufacturing processes and service status. NMR is uniquely placed toinspect the manufacturing process for response to the thermal treatmentapplied to polymerise the molecular structure of plastic matrices. Thedifficulties of correctly treating large, complex shaped pieces aresubstantial and for critical airframe components, must be reliablyovercome. It has been shown, for example, that water can eventuallyinfiltrate polymer structures and modify mechanical performance. Again,NMR is uniquely placed to provide chemical information that quantifiesthe extent of solvent ingression. Furthermore, many manufacturers in thepolymer manufacturing business need to develop new fabrication andassembly techniques for complex components. This has been difficult toachieve up until now due to the unavailability of suitablenon-destructive testing instruments.

In accordance with a second aspect of the present invention, a method ofinspecting a structure comprises generating a set of substantiallyplanar or curved regions in each of which a magnetic field is generatedwhich is substantially homogeneous, the magnitude of the field withineach region differing between the regions; positioning a structure to beinspected such that it intersects at least one of the homogeneousworking regions; exposing the structure to a RF pulse at a predeterminedfrequency band to excite nuclear magnetic resonance within thestructure; and monitoring the resulting signal echo.

With this method, large polymer composites such as airframe componentsand the like can be inspected using nuclear magnetic resonance and itshould be noted that in contrast to known NMR apparatus whichsuperimposes magnetic field gradients in order to define that part ofthe structure which is to be examined, the invention can be used toachieve spatial resolution by suitably choosing the RF frequency.

Typically, the method will be performed using a magnetic fieldgenerating assembly according to the first aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of a method and apparatus according to the invention will nowbe described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of the apparatus;

FIG. 2 illustrates graphically the variation in second and fourth orderradial derivatives with coil radius of the magnetic field due to asingle coil.

FIG. 3 illustrates graphically an example of a coil assembly accordingto the invention;

FIG. 4 illustrates graphically another example of an assembly accordingto the invention.

FIG. 5 illustrates a typical pulse sequence for use with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an example of NMR apparatus in schematic form whichcomprises a coil assembly 1 described in more detail below in connectionwith FIG. 4. The apparatus comprises a pair of coaxial, superconductingcoils 2,3 spaced apart along a common axis 4. The coils 2,3 will behousedin a conventional cryostat (not shown). As explained below, thecoils are positioned so as to generate in a working region 5 a magneticfield which exhibits a linear gradient along the axis 4 and issubstantially homogeneous in the direction orthogonal to the axis 4.Consequently, the working region 5 defines a region suitable forperforming an NMR experiment. The apparatus includes an RF coil 6positioned to generate an RF magnetic field orthogonal to the directionof the magnetic field withinthe working region and also to detectresultant signal echoes. Finally, conventional gradient coils 7 may beprovided to superimpose gradients in directions orthogonal to the axisor within the working region 5.

The invention will be described by way of example in connection with aworking region 5 situated 15 cm away from the nearest part of the coil2. This distance will provide space for mechanical supports and, in thecase of superconducting coils, cryogenic components. A system will bedescribedsuch that the radial extent of the working region is about 30cm so as to provide an area of more than 0.25 m².

It is necessary to consider the radial variation of field at the pointof interest. The field at a point R on the surface of the working region5 can be written as a Taylor expansion about the axis 4 (the Z axis):##EQU1##

Because of the symmetry, only the even terms appear. To obtain a set offlat, evenly spaced surfaces, one should seek a coil system where theradial derivatives sum to zero. The greater the number of derivativesone can make sufficiently small, the larger will be the radial extent ofthe working volume.

The calculation of the radial derivatives is not so straightforward asfor the axial ones. Where computer programs for calculating off-axisfields are available, the radial derivatives may be obtained either byfitting a polynomial to a radial field plot, or by numericaldifferentiation. These techniques are known to people skilled in the artand will not be descriedin detail. FIG. 2 shows the second and fourthradial derivatives for a coil8 (8000 turn/cm²) plotted as a function ofthe radius of the coil. It can be seen that the second derivative goesthrough zero and changes sign from negative to positive at about aradius of 35 cm in this case. The fourth derivative changes sign twiceat about 12 cm and 65 cm. This gives us sufficient degrees of freedom tocancel with the region 5 the second derivatives with a pair of coils orto cancel both second and fourth derivatives simultaneously with asystem of three coils. The data used to generate FIG. 2 is given inTable 1 below. In this Table, the dimensions a1, b1, a2 and b2 are asshown in FIG. 2 while the zero to 4th order derivatives of axial fieldare denoted by H0-H4 respectively.

                  TABLE 1                                                         ______________________________________                                        Coil Position 1                                                               a1           cm          1.000E+001                                           a2                       2.000E+001                                           b1                      -2.500E+001                                           b2                      -1.500E+001                                           highest order       4.000E+000                                                Radial derivatives gauss, cm units                                            H0 = 7.358E+003     H1 = 0.000E+000                                           H2 = -3.777E+001    H3 = 0.000E+000                                           H4 = 2.010E-001                                                               Coil Position 2                                                               a1           cm          2.000E+001                                           a2                       3.000E+001                                           b1                      -2.500E+001                                           b2                      -1.500E+001                                           highest order       4.000E+000                                                H0 = 9.587E+003     H1 = 0.000E+000                                           H2 = -1.329E+001    H3 = 0.000E+000                                           H4 = -1.899E-001                                                              Coil Position 3                                                               a1           cm          3.000E+001                                           a2                       4.000E+001                                           b1                      -2.500E+001                                           b2                      -1.500E+001                                           highest order       4.000E+000                                                H0 = 9.393E+003     H1 = 0.000E+000                                           H2 = -2.139E+000    H3 = 0.000E+000                                           H4 = -7.542E-002                                                              Coil Position 4                                                               a1           cm          4.000E+001                                           a2                       5.000E+001                                           b1                      -2.500E+001                                           b2                      -1.500E+001                                           highest order       4.000E+000                                                H0 = 8.515E+003     H1 = 0.000E+000                                           H2 = 8.395E-001     H3 = 0.000E+000                                           H4 = -2.158E-002                                                              Coil Position 5                                                               a1           cm          5.000E+001                                           a2                       6.000E+001                                           b1                      -2.500E+001                                           b2                      -1.500E+001                                           highest order       4.000E+000                                                H0 = 7.579E+003     H1 = 0.000E+000                                           H2 = 1.350E+000     H3 = 0.000E+000                                           H4 = -5.393E-003                                                              Coil Position 6                                                               a1           cm          6.000E+001                                           a2                       7.000E+001                                           b1                      -2.500E+001                                           b2                      -1.500E+001                                           highest order       4.000E+000                                                H0 = 6.747E+003     H1 = 0.000E+000                                           H2 = 1.237E+000     H3 = 0.000E+000                                           H4 = -9.096E-004                                                              Coil Position 7                                                               a1           cm          7.000E+001                                           a2                       8.000E+001                                           b1                      -2.500E+001                                           b2                      -1.500E+ 001                                          highest order       4.000E+000                                                H0 = 6.042E+003     H1 = 0.000E+000                                           H2 = 1.011E+000     H3 = 0.000E+000                                           H4 = 2.356E-004                                                               Coil Position 8                                                               a1           cm          8.000E+001                                           a2                       9.000E+001                                           b1                      -2.500E+001                                           b2                      -1.500E+001                                           highest order       4.000E+000                                                H0 = 5.452E+003     H1 = 0.000E+000                                           H2 = 8.013E-001     H3 = 0.000E+000                                           H4 = 4.457E-004                                                               Coil Position 9                                                               a1           cm          9.000E+001                                           a2                       1.000E+002                                           b1                      -2.500E+001                                           b2                      -1.500E+001                                           highest order       4.000E+000                                                H0 = 4.956E+003     H1 = 0.000E+000                                           H2 = 6.322E-001     H3 = 0.000E+000                                           H4 = 4.101E-004                                                               Coil Position 10                                                              a1           cm          1.000E+002                                           a2                       1.100E+002                                           b1                      -2.500E+001                                           b2                      -1.500E+001                                           highest order       4.000E+000                                                H0 = 4.536E+003     H1 = 0.000E+000                                           H2 = 5.016E-001     H3 = 0.000E+000                                           H4 = 3.266E-004                                                               ______________________________________                                    

FIG. 3 shows a simple example of a two-coil system formed by coils 9,10.FIG. 3 has two sections, the main section illustrating contours ofconstant field labelled 11-13 corresponding to field values of10,001,8784, and 7545 gauss respectively while the second section ofFIG. 3 illustrates the variation in total field strength along the axisof the assembly at different radii. Each coil has a turn density of 8000amp/cm². The data used to generate FIG. 3 is given in Table 2 below. Inthis Table, the values Hmod are plotted, where Hmod² =Hr² +Hz², and Hr,Hz are the radial and axial field components respectively. It will beappreciated that only some of the field plots areshown in FIG. 3.

                                      TABLE 2                                     __________________________________________________________________________    Coil 10                                                                       a1      cm        3.400E+001                                                  a2                3.900E+001                                                  b1               -2.500E+001                                                  b2               -1.500E+001                                                  highest order    4.000/E+000                                                  Coil 9                                                                        a1      cm        4.500E+001                                                  a2                5.000E+001                                                  b1               -2.500E+001                                                  b2               -1.500E+001                                                  highest order    4.000E+000                                                   Radial derivatives                                                            H0 = 8.748E+003  H1 = 0.000E+000                                              H2 = -3.996E-002 H3 = 0.000E+000                                              H4 = -3.823E-002                                                              Field plots                                                                   Z        Hr        hz        Hmod                                             __________________________________________________________________________    R 0.000E+000                                                                  -1.000E+001                                                                             0.000E+000                                                                              1.109E+004                                                                             1.109E+004                                       -7.500E+000                                                                             0.000E+000                                                                              1.058E+004                                                                             1.058E+004                                       -5.000E+000                                                                             0.000E+000                                                                              1.001E+004                                                                             1.001E+004                                       -2.500E+000                                                                             0.000E+000                                                                              9.408E+003                                                                             9.408E+003                                        0.000E+000                                                                             0.000E+000                                                                              8.784E+000                                                                             8.784E+003                                        2.500E+000                                                                             0.000E+000                                                                              8.158E+003                                                                             8.158E+003                                        5.000E+000                                                                             0.000E+000                                                                              7.545E+003                                                                             7.545E+003                                        7.500E+000                                                                             0.000E+ 000                                                                             6.953E+003                                                                             6.953E+003                                        1.000E+001                                                                             0.000E+000                                                                              6.390E+003                                                                             6.390E+003                                       R 5.000E+000                                                                  -1.000E+001                                                                             4.801E+002                                                                              1.117E+004                                                                             1.118E+004                                       -7.500E+000                                                                             5.520E+002                                                                              1.064E+004                                                                             1.066E+004                                       -5.000E+000                                                                             6.005E+002                                                                              1.005E+004                                                                             1.007E+004                                       -2.500E+000                                                                             6.273E+002                                                                              9.426E+003                                                                             9.447E+003                                        0.000E+000                                                                             6.352E+002                                                                              8.785E+003                                                                             8.808E+003                                        2.500E+000                                                                             6.277E+002                                                                              8.145E+003                                                                             8.169E+003                                        5.000E+000                                                                             6.085E+002                                                                              7.521E+003                                                                             7.546E+003                                        7.500E+000                                                                             5.808E+002                                                                              6.923E+003                                                                             6.947E+003                                        1.000E+001                                                                             5.476E+002                                                                              6.356E+003                                                                             6.379E+003                                       R 1.000E+001                                                                  -1.000E+001                                                                             1.032E+003                                                                              1.142E+004                                                                             1.147E+004                                       -7.500E+000                                                                             1.176E+003                                                                              1.082E+004                                                                             1.088E+004                                       -5.000E+000                                                                             1.268E+003                                                                              1.016E+004                                                                             1.024E+004                                       -2.500E+000                                                                             1.312E+003                                                                              9.409E+003                                                                             9.559E+003                                        0.000E+000                                                                             1.317E+003                                                                              8.775E+003                                                                             8.873E+003                                        2.500E+000                                                                             1.290E+003                                                                              8.095E+003                                                                             8.198E+003                                        5.000E+000                                                                             1.242E+003                                                                              7.442E+003                                                                             7.545E+ 003                                       7.500E+000                                                                             1.177E+003                                                                              6.824E+003                                                                             6.925E+003                                        1.000E+000                                                                             1.104E+003                                                                              6.246E+003                                                                             6.343E+003                                       R 1.500E+001                                                                  -1.000E+001                                                                             1.756E+003                                                                              1.186E+004                                                                             1.199E+004                                       -7.500E+000                                                                             1.966E+003                                                                              1.110E+004                                                                             1.127E+004                                       -5.000E+000                                                                             2.081E+003                                                                              1.030E+004                                                                             1.051E+004                                       -2.500E+000                                                                             2.117E+003                                                                              9.500E+003                                                                             9.733E+003                                        0.000E+000                                                                             2.090E+003                                                                              8.718E+003                                                                             8.965E+003                                        2.500E+000                                                                             2.019E+003                                                                              7.974E+003                                                                             8.225E+003                                        5.000E+000                                                                             1.918E+003                                                                              7.277E+003                                                                             7.526E+003                                        7.500E+000                                                                             1.800E+003                                                                              6.633E+003                                                                             6.873E+003                                        1.000E+001                                                                             1.672E+003                                                                              6.042E+003                                                                             6.269E+003                                       R 2.000E+001                                                                  -1.000E+001                                                                             2.819E+003                                                                              1.246E+004                                                                             1.277E+004                                       -7.500E+000                                                                             3.059E+003                                                                              1.143E+004                                                                             1.183E+004                                       -5.000E+000                                                                             3.141E+003                                                                              1.041E+004                                                                             1.087E+004                                       -2.500E+000                                                                             3.100E+003                                                                              9.435E+003                                                                             9.934E+003                                        0.000E+000                                                                             2.993E+003                                                                              8.535E+003                                                                             9.045E+003                                        2.500E+000                                                                             2.831E+003                                                                              7.715E+003                                                                             8.218E+003                                        5.000E+000                                                                              2.642E+003                                                                             6.975E+003                                                                             7.459E+003                                        7.500E+000                                                                             2.443E+003                                                                              6.311E+003                                                                             6.767E+003                                        1.000E+001                                                                             2.242E+003                                                                              5.717E+003                                                                             6.141E+003                                       R 2.500E+001                                                                  -1.000E+001                                                                             4.532E+003                                                                              1.311E+004                                                                             1.387E+004                                       -7.500E+000                                                                             4.653E+003                                                                              1.162E+004                                                                             1.251E+004                                       -5.000E+000                                                                             4.549E+003                                                                              1.028E+004                                                                             1.124E+004                                       -2.500E+000                                                                             4.318E+003                                                                              9.108E+003                                                                             1.008E+004                                        0.000E+000                                                                             4.023E+003                                                                              8.097E+003                                                                             9.041E+003                                        2.500E+000                                                                             3.705E+003                                                                              7.224E+003                                                                             8.119E+003                                        5.000E+000                                                                             3.385E+003                                                                              7.224E+003                                                                             7.301E+003                                        7.500E+000                                                                             3.076E+003                                                                              5.812E+003                                                                             6.576E+003                                        1.000E+001                                                                             2.785E+003                                                                              5.240E+003                                                                             5.934E+003                                       R 3.000E+001                                                                  -1.000E+001                                                                             7.388E+003                                                                              1.320E+004                                                                             1.512E+004                                       -7.500E+000                                                                             6.889E+003                                                                              1.111E+004                                                                             1.307E+004                                       -5.000E+000                                                                             6.275E+003                                                                              9.514E+003                                                                             1.140E+004                                       -2.500E+000                                                                             5.658E+003                                                                              8.254E+003                                                                             1.001E+004                                        0.000E+000                                                                             5.077E+003                                                                              7.237E+003                                                                             8.840E+003                                        2.500E+000                                                                             4.545E+003                                                                              6.399E+003                                                                             7.849E+003                                        5.000E+000                                                                             4.065E+003                                                                              5.698E+003                                                                             6.999E+003                                        7.500E+000                                                                             3.634E+003                                                                              5.104E+003                                                                             6.265E+003                                        1.000E+001                                                                             3.249E+003                                                                              4.595E+003                                                                             5.628E+003                                       Contour plots                                                                 R        Z         dB/dZ     error                                            __________________________________________________________________________    Z -5.000E+000                                                                  0.000E+000                                                                            -5.000E+000                                                                             -2.345E+002                                                                             1.001E+004                                        2.500E+000                                                                            -4.941E+000                                                                             -2.367E+002                                                                             8.387E-006                                        5.000E+000                                                                            -4.770E+000                                                                             -2.431E+002                                                                             2.506E-005                                        7.500E+000                                                                            -4.504E+000                                                                             -2.537E+002                                                                             4.037E-005                                        1.000E+000                                                                            -4.168E+000                                                                             -2.685E+002                                                                             5.354E-005                                        1.250E+000                                                                            -3.789E+000                                                                             -2.874E+002                                                                             6.261E-005                                        1.500E+000                                                                            -3.401E+000                                                                             -3.102E+002                                                                             6.436E-005                                        1.750E+000                                                                            -3.034E+000                                                                             -3.367E+002                                                                             5.890E-005                                        2.000E+000                                                                            -2.718E+000                                                                             -3.669E+002                                                                             4.281E-005                                        2.250E+000                                                                            -2.481E+000                                                                             -4.004E+002                                                                             1.921E-005                                        2.500E+000                                                                            -2.351E+000                                                                             -4.365E+002                                                                             2.340E-006                                        2.750E+000                                                                            -2.353E+000                                                                             -4.743E+002                                                                             1.950E-007                                        3.000E+000                                                                            -2.512E+000                                                                             -5.117E+002                                                                             5.715E-005                                       Z 0.000E+000                                                                   0.000E+000                                                                             0.000E+ 000                                                                            -2.500E+002                                                                             8.784E+003                                        2.500E+000                                                                             2.358E-002                                                                             -2.514E+002                                                                             3.002E-006                                        5.000E+000                                                                             9.162E-002                                                                             -2.555E+002                                                                             8.116E-006                                        7.500E+000                                                                             1.966E-001                                                                             -2.623E+002                                                                             1.223E-005                                        1.000E+000                                                                             3.268E-001                                                                             -2.718E+002                                                                             1.490E-005                                        1.250E+000                                                                             4.673E-001                                                                             -2.839E+002                                                                             1.534E-005                                        1.500E+000                                                                             6.004E-001                                                                             -2.986E+002                                                                             1.323E-005                                        1.750E+000                                                                             7.071E-001                                                                             -3.158E+002                                                                             8.560E-006                                        2.000E+000                                                                             7.674E-001                                                                             -3.353E+002                                                                             3.002E-006                                        2.250E+000                                                                             7.610E-001                                                                             -3.569E+002                                                                             1.112E-007                                        2.500E+000                                                                             6.672E-001                                                                             -3.800E+002                                                                             7.115E-006                                        2.750E+000                                                                             4.651E-001                                                                             -4.040E+002                                                                             3.491E-005                                        3.000E+000                                                                             1.331E-001                                                                             -4.278E+002                                                                             9.272E-005                                       Z 5.000E+000                                                                   0.000E+000                                                                             5.000E+000                                                                             -2.411E+002                                                                             7.545E+003                                        2.500E+000                                                                             5.002E+000                                                                             -2.420E+002                                                                             6.472E-008                                        5.000E+000                                                                             5.006E+000                                                                             -2.445E+002                                                                             3.236E-007                                        7.500E+000                                                                             5.008E+000                                                                             -2.487E+002                                                                             1.942E-007                                        1.000E+000                                                                             5.002E+000                                                                             -2.545E+002                                                                             3.883E-007                                        1.250E+000                                                                             4.980E+000                                                                             -2.620E+002                                                                             1.294E-006                                        1.500E+000                                                                             4.931E+000                                                                             -2.710E+002                                                                             1.877E-006                                        1.750E+000                                                                             4.845E+000                                                                             -2.815E+002                                                                             1.489E-006                                        2.000E+000                                                                             4.707E+000                                                                             -2.934E+002                                                                             1.489E-006                                        2.250E+000                                                                             4.505E+000                                                                             -3.064E+002                                                                             1.016E-005                                        2.500E+000                                                                             4.223E+000                                                                             -3.203E+002                                                                             2.796E-005                                        2.750E+000                                                                             3.845E+000                                                                             -3.348E+002                                                                             5.889E-005                                        3.000E+000                                                                             3.355E+000                                                                             -3.492E+002                                                                             1.058E-004                                       __________________________________________________________________________

As can be seen in FIG. 3, the working region 5 has substantially flatsurfaces effectively defined by the 10,001 and 7545 gauss contours. Thissystem is dominated by the fourth derivative which could be cancelled bymaking use of a third coil. However, it is possible to exploit thevariation of the radial gradients with the axial position of the coilsto achieve this without the need for a third coil. This is illustratedin FIG. 4 which is similar to FIG. 3 but for a system such as that shownschematically in FIG. 1 in which coils 11, 12 are offset along a commonaxis. This achieves the same cancellation of second order radialgradientsas in FIG. 3 but has a lower fourth derivative due to thegreater axial distance of the inner coil from the working region.

The data used to generate FIG. 4 is set out in Table 3 below.

                                      TABLE 3                                     __________________________________________________________________________    Coil 11                                                                       a1      cm        3.400E+001                                                  a2                3.610E+001                                                  b1               -3.500E+001                                                  b2               -2.500E+001                                                  highest order    4.000/E+000                                                  Coil 12                                                                       a1      cm        4.500E+001                                                  a2                5.350E+001                                                  b1               -2.500E+001                                                  b2               -1.500E+001                                                  highest order    4.000E+000                                                   Radial derivatives                                                            H0 = 8.221E+003  H1 = 0.000E+000                                              H2 = -1.915E-002 H3 = 0.000E+000                                              H4 = -1.581E-002                                                              Field plots                                                                   Z        Hr        Hz        Hmod                                             __________________________________________________________________________    R 0.000E+000                                                                  -1.000E+001                                                                             0.000E+000                                                                              1.012E+004                                                                             1.012E+004                                       -7.500E+000                                                                             0.000E+000                                                                              9.685E+003                                                                             9.685E+004                                       -5.000E+000                                                                             0.000E+000                                                                              9.214E+003                                                                             9.214E+004                                       -2.500E+000                                                                             0.000E+000                                                                              8.723E+003                                                                             8.723E+003                                        0.000E+000                                                                             0.000E+000                                                                              8.221E+003                                                                             8.221E+003                                        2.500E+000                                                                             0.000E+000                                                                              7.719E+003                                                                             7.719E+003                                        5.000E+000                                                                             0.000E+000                                                                              7.223E+003                                                                             7.223E+003                                        7.500E+000                                                                             0.000E+ 000                                                                             6.740E+003                                                                             6.740E+003                                        1.000E+001                                                                             0.000E+000                                                                              6.273E+003                                                                             6.273E+003                                       R 5.000E+000                                                                  -1.000E+001                                                                             4.252E+002                                                                              1.017E+004                                                                             1.018E+004                                       -7.500E+000                                                                             4.628E+002                                                                              9.716E+003                                                                             9.727E+004                                       -5.000E+000                                                                             4.883E+002                                                                              9.234E+003                                                                             9.247E+004                                       -2.500E+000                                                                             5.027E+002                                                                              8.732E+003                                                                             8.747E+003                                        0.000E+000                                                                             5.074E+002                                                                              8.222E+003                                                                             8.238E+003                                        2.500E+000                                                                             5.037E+002                                                                              7.712E+003                                                                             7.729E+003                                        5.000E+000                                                                             4.933E+002                                                                              7.210E+003                                                                             7.227E+003                                        7.500E+000                                                                             4.775E+002                                                                              6.722E+003                                                                             6.739E+003                                        1.000E+001                                                                             4.576E+002                                                                              6.252E+003                                                                             6.269E+003                                       R 1.000E+001                                                                  -1.000E+001                                                                             8.894E+003                                                                              1.030E+004                                                                             1.034E+004                                       -7.500E+000                                                                             9.623E+003                                                                              9.808E+003                                                                             9.855E+003                                       -5.000E+000                                                                             1.010E+003                                                                              9.290E+003                                                                             9.345E+003                                       -2.500E+000                                                                             1.035E+003                                                                              8.757E+003                                                                             8.818E+003                                        0.000E+000                                                                             1.040E+003                                                                              8.219E+003                                                                             8.285E+003                                        2.500E+000                                                                             1.028E+003                                                                              7.687E+003                                                                             7.755E+003                                        5.000E+000                                                                             1.003E+003                                                                              7.167E+003                                                                             7.236E+ 003                                       7.500E+000                                                                             9.669E+003                                                                              6.665E+003                                                                             6.735E+003                                        1.000E+000                                                                             9.235E+003                                                                              6.185E+003                                                                             6.254E+003                                       R 1.500E+001                                                                  -1.000E+001                                                                             1.436E+003                                                                              1.051E+004                                                                             1.061E+004                                       -7.500E+000                                                                             1.539E+003                                                                              9.951E+003                                                                             1.007E+004                                       -5.000E+000                                                                             1.601E+003                                                                              9.371E+003                                                                             9.507E+003                                       -2.500E+000                                                                             1.627E+003                                                                              8.783E+003                                                                             8.933E+003                                        0.000E+000                                                                             1.622E+003                                                                              8.199E+003                                                                             8.358E+003                                        2.500E+000                                                                             1.592E+003                                                                              7.629E+003                                                                             7.794E+003                                        5.000E+000                                                                             1.543E+003                                                                              7.080E+003                                                                             7.246E+003                                        7.500E+000                                                                             1.478E+003                                                                              6.557E+003                                                                             6.721E+003                                        1.000E+001                                                                             1.404E+003                                                                              6.063E+003                                                                             6.223E+003                                       R 2.000E+001                                                                  -1.000E+001                                                                             2.120E+003                                                                              1.079E+004                                                                             1.099E+004                                       -7.500E+000                                                                             2.241E+003                                                                              1.013E+004                                                                             1.037E+004                                       -5.000E+000                                                                             2.302E+003                                                                              9.452E+003                                                                             9.728E+003                                       -2.500E+000                                                                             2.312E+003                                                                              8.784E+003                                                                             9.083E+003                                        0.000E+000                                                                             2.280E+003                                                                              8.135E+003                                                                             8.448E+003                                        2.500E+000                                                                             2.215E+003                                                                              7.513E+003                                                                             7.833E+003                                        5.000E+000                                                                              2.125E+003                                                                             6.926E+003                                                                             7.245E+003                                        7.500E+000                                                                             2.019E+003                                                                              6.377E+003                                                                             6.688E+003                                        1.000E+001                                                                             1.902E+003                                                                              5.867E+003                                                                             6.167E+003                                       R 2.500E+001                                                                  -1.000E+001                                                                             3.007E+003                                                                              1.111E+004                                                                             1.151E+004                                       -7.500E+000                                                                             3.125E+003                                                                              1.029E+004                                                                             1.076E+004                                       -5.000E+000                                                                             3.160E+003                                                                              9.490E+003                                                                             1.000E+004                                       -2.500E+000                                                                             3.125E+003                                                                              8.714E+003                                                                             9.257E+003                                        0.000E+000                                                                             3.036E+003                                                                              7.980E+003                                                                             8.538E+003                                        2.500E+000                                                                             2.908E+003                                                                              7.297E+003                                                                             7.855E+003                                        5.000E+000                                                                             2.755E+003                                                                              6.668E+003                                                                             7.215E+003                                        7.500E+000                                                                             2.586E+003                                                                              6.094E+003                                                                             6.620E+003                                        1.000E+001                                                                             2.411E+003                                                                              5.573E+003                                                                             6.072E+003                                       R 3.000E+001                                                                  -1.000E+001                                                                             4.188E+003                                                                              1.143E+004                                                                             1.217E+004                                       -7.500E+000                                                                             4.273E+003                                                                              1.040E+004                                                                             1.124E+004                                       -5.000E+000                                                                             4.234E+003                                                                              9.409E+003                                                                             1.032E+004                                       -2.500E+000                                                                             4.102E+003                                                                              8.492E+003                                                                             9.431E+003                                        0.000E+000                                                                             3.906E+003                                                                              7.660E+003                                                                             8.598E+003                                        2.500E+000                                                                             3.672E+003                                                                              6.916E+003                                                                             7.830E+003                                        5.000E+000                                                                             3.420E+003                                                                              6.255E+003                                                                             7.129E+003                                        7.500E+000                                                                             3.164E+003                                                                              5.670E+003                                                                             6.493E+003                                        1.000E+001                                                                             2.913E+003                                                                              5.153E+003                                                                             5.919E+003                                       Contour plots                                                                 R        Z         dB/dZ     error                                            __________________________________________________________________________    Z -5.000E+000                                                                  0.000E+000                                                                            -5.000E+000                                                                             -1.924E+002                                                                             9.214E+003                                        2.500E+000                                                                            -4.957E+000                                                                             -1.934E+002                                                                             3.498E-006                                        5.000E+000                                                                            -4.832E+000                                                                             -1.966E+002                                                                             1.017E-005                                        7.500E+000                                                                            -4.633E+000                                                                             -2.018E+002                                                                             1.600E-005                                        1.000E+000                                                                            -4.371E+000                                                                             -2.090E+002                                                                             2.141E-005                                        1.250E+000                                                                            -4.061E+000                                                                             -2.182E+002                                                                             2.565E-005                                        1.500E+000                                                                            -3.720E+000                                                                             -2.292E+002                                                                             2.851E-005                                        1.750E+000                                                                            -3.363E+000                                                                             -2.421E+002                                                                             3.021E-005                                        2.000E+000                                                                            -3.006E+000                                                                             -2.568E+002                                                                             3.063E-005                                        2.250E+000                                                                            -2.664E+000                                                                             -2.734E+002                                                                             3.042E-005                                        2.500E+000                                                                            -2.351E+000                                                                             -2.923E+002                                                                             2.862E-005                                        2.750E+000                                                                            -2.080E+000                                                                             -3.138E+002                                                                             2.512E-005                                        3.000E+000                                                                            -1.865E+000                                                                             -3.383E+002                                                                             1.812E-005                                       Z 0.000E+000                                                                   0.000E+000                                                                             0.000E+ 000                                                                            -2.007E+002                                                                             8.221E+003                                        2.500E+000                                                                             2.034E-002                                                                             -2.015E+002                                                                             1.782E-006                                        5.000E+000                                                                             7.918E-002                                                                             -2.037E+002                                                                             1.402E-005                                        7.500E+000                                                                             1.748E-001                                                                             -2.073E+002                                                                             6.889E-006                                        1.000E+000                                                                             2.989E-001                                                                             -2.124E+002                                                                             8.196E-006                                        1.250E+000                                                                             4.442E-001                                                                             -2.189E+002                                                                             9.740E-006                                        1.500E+000                                                                             6.015E-001                                                                             -2.268E+002                                                                             1.045E-005                                        1.750E+000                                                                             7.605E-001                                                                             -2.362E+002                                                                             1.033E-005                                        2.000E+000                                                                             9.107E-001                                                                             -2.470E+002                                                                             9.384E-006                                        2.250E+000                                                                             1.041E+000                                                                             -2.595E+002                                                                             7.721E-006                                        2.500E+000                                                                             1.141E+000                                                                             -2.736E+002                                                                             5.464E-006                                        2.750E+000                                                                             1.198E+000                                                                             -2.896E+002                                                                             2.376E-006                                        3.000E+000                                                                             1.199E+000                                                                             -3.077E+002                                                                             1.188E-007                                       Z 5.000E+000                                                                   0.000E+000                                                                             5.000E+000                                                                             -1.959E+002                                                                             7.223E+003                                        2.500E+000                                                                             5.005E+000                                                                             -1.964E+002                                                                             2.028E-007                                        5.000E+000                                                                             5.019E+000                                                                             -1.979E+002                                                                             6.760E-007                                        7.500E+000                                                                             5.041E+000                                                                             -2.004E+002                                                                             1.082E-006                                        1.000E+000                                                                             5.066E+000                                                                             -2.040E+002                                                                             1.284E-006                                        1.250E+000                                                                             5.090E+000                                                                             -2.085E+002                                                                             1.149E-006                                        1.500E+000                                                                             5.107E+000                                                                             -2.141E+002                                                                             6.760E-007                                        1.750E+000                                                                             5.111E+000                                                                             -2.207E+002                                                                             6.760E-008                                        2.000E+000                                                                             5.095E+000                                                                             -2.284E+002                                                                             6.760E-007                                        2.250E+000                                                                             5.049E+000                                                                             -2.372E+002                                                                             1.217E-006                                        2.500E+000                                                                             4.996E+000                                                                             -2.472E+002                                                                             1.284E-006                                        2.750E+000                                                                             4.837E+000                                                                             -2.585E+002                                                                             2.028E-007                                        3.000E+000                                                                             4.650E+000                                                                             -2.711E+002                                                                             4.732E-006                                       __________________________________________________________________________

In order to carry out an NMR experiment within the working region 5, itcanbe desirable to remove dynamically the static magnetic field gradientwhichexists in the axial direction. This can be done in a variety ofways such as that illustrated by way of example with reference to FIG. 5in connection with spin echo imaging. In the particular case shown, thesliceselection direction is chosen to be in the axial direction relativeto the main magnetic field. This sequence comprises a frequency (andtherefore spatially) selective 90 degree RF pulse P1, followed by anon-selective re-phasing 180 degree pulse, P2, which create echo E1. Thedynamically applied gradient sequence is labelled "Applied ReadGradient", "Applied Phase Encode Gradient" and "Applied Slice Gradient".Taking these in order, the Read Gradient is applied in the radialdirection and is essentially the same as is required within the bore ofa conventional imaging magnet. In this direction, the spin system isde-phased after the selective pulse P1 and re-phased after the nonselective pulse P2 to form the echo which is detected and digitized asnormal. Likewise, the phase encode gradient is applied between the twopulses, P1 and P2, and its amplitude is varied in each successiveapplication to provide a second dimension of spatial encoding and thus afull two dimensional image. The applied Slice Gradient produces thedesired slice gradient in conjunction with the static field gradient,the static gradient being inherent to the main magnetic field system.

In detail, at time marked A no gradient is dynamically applied duringthe course of the selective radio frequency pulse P1 which is designedto rotate just the selected spins. After the pulse P1 is over, agradient pulse is applied between points marked B and C, to rephase thespins to produce a coherent signal. Typically, as is well known to thoseskilled inthe art, this gradient must be in the reverse direction tothat present during the pulse. If the pulse P1 is both amplitude and orphase modulatedthe rotated spins may still be in phase following RFexcitation in which case the gradient portion B to C will not berequired to be different to the amplitude during period C to D.Following point C, a gradient field isapplied throughout the remainderof the sequence until the echo has been sampled at point D, and then canbe removed until a subsequent sequence.

As with conventional imaging, a selective pulse can be used in place ofP2,again with the appropriate offset added to the conventional slicegradient during the selective 180 degree RF re-phase process. In thisway, a multi-slice imaging procedure can be performed by theconventional technique of changing the radio frequency for each slice.

The same principle can be applied to the read and phase encodegradients. The same style of addition of static offset should be appliedto any gradient pulses that are applied in the axial direction.

We claim:
 1. A magnetic field generating assembly comprising:at leasttwo magnetic field generators for generating magnetic fields in the samesense, the generators being arranged such that in use in a workingregion external to the generators the resultant magnetic field exhibitsa gradient extending in a first direction, the generators beingsubstantially coaxial electrical coils with the axis extending in thefirst direction, and wherein at least second order variations in thefields in directions transverse to the first direction within the regionare substantially balanced such that an NMR experiment can be performedon a sample within the working region.
 2. An assembly according to claim1, wherein the centres of the coils are offset along the axis.
 3. Anassembly according to claim 1, wherein the working region issubstantially planar.
 4. An assembly according to claim 1, wherein theworking region is substantially dish shaped.
 5. An assembly according toclaim 1, wherein homogeneity of the field in the working region indirections transverse to the first direction is better than 100 ppm. 6.An assembly according to claim 1, wherein at least second and fourthorder variations in the fields in directions transverse to the firstdirection are substantially balanced in the working region.
 7. Apparatusfor performing an NMR experiment, the apparatus comprising:an assemblyhaving at least two magnetic field generators for generating magneticfields in the same sense, the generators being arranged such that in usein a working region external to the generators the resultant magneticfield exhibits a gradient extending in a first direction, the generatorsbeing substantially coaxial electrical coils with the axis extending inthe first direction, and wherein at least second order variations in thefields in directions transverse to the first direction within the regionare substantially balanced such that an NMR experiment can be performedon a sample within the working region; RF magnetic field generatingmeans for generating an RF magnetic field in the working region; anddetection means for detecting the resultant signal echo.
 8. Apparatusaccording to claim 7, further comprising gradient magnetic fieldgenerating means for generating magnetic field gradients in the workingregion transverse to the first direction.
 9. An assembly according toclaim 7, wherein the working region is substantially planar.
 10. Anassembly according to claim 7, wherein the working region issubstantially dish shaped.
 11. An apparatus according to claim 7,wherein homogeneity of the field in the working region in directionstransverse to the first direction is better than 100 ppm.
 12. Anapparatus according to claim 7, wherein at least second and fourth ordervariations in the fields in directions transverse to the first directionare substantially balanced in the working region.
 13. A method ofinspecting a structure comprising:(a) generating magnetic fields in thesame sense by at least two magnetic field generators, the generatorsbeing arranged such that in use in a working region external to thegenerators the resultant magnetic field exhibits a gradient extending ina first direction, the generators being substantially coaxial electricalcoils with the axis extending in the first direction, and wherein atleast second order variations in the fields in directions transverse tothe first direction within the working region are substantially balancedsuch that inspection of a structure can be performed on a sample withinthe working region; (b) generating a set of substantially planar orcurved regions in each of which a magnetic field is generated which issubstantially homogeneous, the magnitude of the field within each regiondiffering between the regions; (c) positioning a structure to beinspected such that it intersects at least one of the homogeneousregions; (d) exposing the structure to a RF pulse at a predeterminedfrequency band to excite the nuclear magnetic resonance within thestructure; and (e) monitoring the resulting signal echo.
 14. A methodaccording to claim 13, further comprising gradient magnetic fieldgenerating means for generating magnetic field gradients in the workingregion transverse to the first direction.